1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using the same. For example, the present invention is suitable for an image forming apparatus such as a laser beam printer (LBP), a digital copying machine, or a multi-function printer, which adopts an electrophotography process.
2. Description of the Related Art
Various color image forming apparatuses have conventionally been proposed, each of which includes light source unit, multiple scanning optical systems having imaging optical systems, and deflection unit common among the multiple scanning optical systems, and guides light beams emitted from the multiple scanning optical systems onto multiple corresponding image bearing members to form a color image (see Japanese Patent Application Laid-Open No. 2004-21133, Japanese Patent Application Laid-Open No. 2004-70107, and Japanese Patent Application Laid-Open No. 2005-338573).
Referring to FIG. 6, main components and optical operations of the above-mentioned conventional color image forming apparatus are described.
In FIG. 6, an image forming unit includes two scanning units SR and SL. The two scanning units SR and SL each have a symmetrical configuration with respect to an optical deflector 28. Thus, the reference numerals are described in FIG. 6 only for one (the scanning unit SR) of the two scanning units.
In the scanning unit SR shown in FIG. 6, a light beam optically modulated based on image information to be emitted travels via a rotational polygon mirror 28 (polygon mirror) which deflects the light beam for scanning, and an imaging optical system LB which has two fθ lenses 29 and 30 which forms an image of the light beam on a photosensitive drum surface 42 (43) in a spot shape. The light beam passed through each of the fθ lenses 29 and 30 travels via a reflecting mirror 31a (31b to 31d) which reflects the light beam in a specific direction and a dust-proof glass 32 (33) which protects the imaging optical system LB from dusts. The light beam passed via the dust-proof glass 32 (33) forms an electrostatic latent image on the photosensitive drum surface 42 (43). The image forming unit includes an optical box 41.
With a progress in configuring an image forming apparatus compact, a system for scanning four photosensitive drums for exposure by one polygon mirror 28 has been adopted in an optical scanning apparatus as illustrated in FIG. 6.
This system includes the two scanning units SR and SL for irradiating opposed surfaces of the polygon mirror 28 with two light beams. The scanning units SR and SL cause two light beams shifted in parallel by a predetermined distance in a sub-scanning direction (vertical direction) to enter a deflecting surface (reflection surface) of the polygon mirror 28 for deflecting the light beams for scanning.
This system includes the two fθ lenses 29 and 30 for imaging the light beams in two upper and lower optical paths on the photosensitive drum surfaces 42 and 43. The two fθ lenses 29 and 30 constitute the imaging optical system LB that includes identical lens surfaces disposed at two upper and lower stages. Its manufacturing method is bonding-together of two lenses or integral molding thereof as a mold lens.
In the scanning unit that uses the imaging optical system LB having the fθ lenses 29 and 30 arranged at the two upper and lower stages, a deflecting surface for deflecting a light beam for scanning with respect to each optical path is necessary for the polygon mirror 28, and a polygon mirror thick in vertical direction or a polygon mirror of a two-stage configuration is used. In this system, since a large polygon mirror is adopted, the load on a motor to drive the polygon mirror is apt to increase.
The above-mentioned conventional color image forming apparatus has had various problems described below.
A first problem is a necessity of an extremely large number of reflecting mirrors for guiding a light beam to the photosensitive drum. For example, in FIG. 6, three reflecting mirrors 31b to 31d are used in the same optical path.
As a result, the apparatus becomes complicated due to the increase in number of components, and the entire apparatus becomes large in size due to the necessity of space to contain the mirrors.
Japanese Patent Application Laid-Open No. 2004-21133 discusses an optical scanning apparatus configured by considering a size of an imaging lens, setting a reflection angle of a reflecting mirror to avoid the imaging lens, and disposing two or one reflecting mirror in the same optical path.
In the optical scanning apparatus discussed in each of Japanese Patent Application Laid-Open No. 2004-21133, Japanese Patent Application Laid-Open No. 2004-70107, and Japanese Patent Application Laid-Open No. 2005-338573, in the optical path for guiding a light beam to an inner surface to be scanned with respect to the optical deflector, a reflection angle of a first mirror is set large to prevent the light beam reflected by the first mirror from being vignetted by the fθ lens. Thus, a position of a second mirror which reflects the reflected light beam from the first mirror is greatly away from the fθ lens, resulting in a larger width of the optical box in a height direction.
Japanese Patent Application Laid-Open No. 2005-33873 discusses a method for reducing a height of an imaging optical system in a sub-scanning direction. In Japanese Patent Application Laid-Open No. 2005-338573, this method tries to reduce a height of an apparatus by focusing on a reflection angle of a reflecting mirror, a distance from an imaging lens to the reflecting mirror, and a height of the imaging lens. In particular, limiting a height of the imaging lens to 6 to 10 mm enables reduction of a folding angle of an optical path.
However, in a lens made of a resin (may be referred to as resin lens hereinafter) generally used for the imaging lens, the following problems arise if a height of the imaging lens is set low.
In the resin lens formed by a mold, when a height of the lens is set low with respect to a thickness of the lens in an optical axis direction, during cooling immediately after taken-out from the mold, the cooling progresses from a vertical direction of the lens. As a result, a refractive index distribution and a birefringence distribution easily occur in the lens within a sub-scanning section (height direction of the lens). Consequently, imaging performance is considerably reduced in a sub-scanning direction. Thus, in the method of Japanese Patent Application Laid-Open No. 2005-338573, reduction of the lens height is difficult.